Touch sensors are transparent or opaque input devices for computers and other electronic systems. As the name suggests, touch sensors are activated by touch, either from a user's finger, or a stylus or some other device. Touchscreens (i.e., touch sensors having a transparent substrate) are generally used in conjunction with display devices, such as cathode ray tube (CRT) monitors and liquid crystal displays, to create touch display systems. These systems are increasingly used in commercial applications such as restaurant order entry systems, industrial process control applications, interactive museum exhibits, public information kiosks, pagers, cellular phones, personal digital assistants, and video games.
The dominant touch technologies presently in use are resistive, capacitive, infrared, and acoustic technologies. Touchscreens incorporating these technologies have delivered high standards of performance at competitive prices. All are transparent devices that respond to a touch by transmitting the touch position coordinates to a host computer, which in turn implements some function associated with the particular position coordinates. Each has, of course, relative strengths and weaknesses.
Acoustic touchscreens, also known as ultrasonic touchscreens, have competed effectively with the other touch technologies. This is due in large part to the ability of acoustic touchscreens to handle demanding applications with high transparency and high resolution touch performance, while providing a durable touch surface.
Acoustic touchscreen systems comprise a touchscreen (i.e., a touch sensor having a transparent substrate), a controller and leads coupling the touchscreen and the controller. Typically, the touchscreen comprises a touch sensitive substrate in which an acoustic wave is propagated. When a touch occurs on the substrate surface, it results in the absorption of at least a portion of the wave energy being propagated across the substrate. Electronic circuitry is used to locate the absorption position in an XY coordinate system that is conceptually and invisibly superimposed onto the touchscreen. In essence, this is accomplished by recording the time the wave is initially propagated and the time at which the touch induced wave absorption occurs. The difference in these times can then be used, together with the known speed of the wave through the substrate, to determine the precise location of the touch.
A common type of acoustic touchscreen employs Rayleigh type acoustic waves—where the term is intended to include quasi-Rayleigh waves. Illustrative disclosures relevant to Rayleigh wave touchscreens include U.S. Pat. Nos. 4,642,423; 4,645,870; 4,700,176; 4,746,914; 4,791,416; Re 33,151; 4,825,212; 4,859,996; 4,880,665; 4,644,100; 5,739,479; 5,708,461; 5,854,450; 5,986,224; 6,091,406; 6,225,985; 6,236,691; and 6,441,809. Acoustic touchscreens employing other types of acoustic waves such as Lamb or shear waves, or combinations of different types of acoustic waves (including combinations involving Rayleigh waves) are also known. Illustrative disclosures of these technologies include U.S. Pat. Nos. 5,591,945; 5,854,450; 5,072,427; 5,162,618; 5,177,327; 5,329,070; 5,573,077; 6,087,599; 5,260,521; and 5,856,820. The above cited patents are hereby incorporated by reference into this application.
Acoustic touchscreens that sense touch via the absorption of Rayleigh waves have proved to be commercially successful. The success of products using Rayleigh waves is due in large part to two properties exhibited by Rayleigh waves. First, Rayleigh waves are more sensitive to touch than are other acoustic waves. Second, Rayleigh waves are surface waves that can propagate on the surface of any simple homogenous glass substrate. However, Rayleigh wave touchscreens have increased sensitivity to liquid contaminants such as oil and water, which absorb energy from the propagating waves.
While Rayleigh waves are most commonly used in commercial products, touchscreens using horizontally polarized shear waves are also well known in the art. The use of horizontally polarized shear waves greatly enhances the robustness of acoustic touchscreen operation in the presence of water and other liquid contaminants. This is due to the fact that unlike Rayleigh waves, horizontally polarized shear waves have no vertical motion component to be absorbed by a contaminant. Therefore, wave absorption occurs through viscous damping rather than wave radiation. Since a finger is more viscous than a contaminant such as a water drop, the touchscreen can be configured to reject lower viscosity touches, thus rejecting contamination, while accepting higher viscosity valid finger touches. Thus, contaminant immunity is an important benefit of horizontally polarized shear waves for certain touchscreen applications.
Whichever type of acoustic technology is used, the acoustic touchscreen comprises transducers, which are elements that convert energy from one form to another. For example, a transmit transducer receives a tone burst from associated electronic circuitry and then emits an acoustic wave packet across a substrate. A receive transducer receives the transmitted acoustic wave packet from the substrate and generates an electronic signal that is transmitted to associated electronic circuitry for processing. Each type of transducer includes a piezoelectric element to transform the electronic signals and mechanical vibrations. Commercial piezoelectric elements are most commonly manufactured from ferroelectric piezoelectric ceramics, such as lead zirconium titanium (PZT) and modified lead titanate. While typically more expensive, mono-crystalline piezoelectric materials, such as lithium niobate, may also be used to construct piezoelectric elements for touchscreen transducers.
Most commercially produced piezoelectric elements are pressure mode piezoelectric elements. However, if the transducer is to transmit or receive a horizontally polarized shear wave, a shear mode piezoelectric element is required. FIGS. 1(a) and (b) schematically show time sequences for piezoelectric element vibrations: FIG. 1(a) shows the time sequence for a pressure mode piezoelectric element and FIG. 1(b) shows the sequence for a shear mode piezoelectric element. In FIG. 1(a), the pressure mode piezoelectric element begins at rest 10. It then receives an electric signal, which causes it to expand to position 12. After the piezoelectric element has reached its fully expanded position 12, it will contract toward its rest position 14. It continues to contract past the rest position 14 until it reaches its fully contracted position 16. Finally, after reaching its fully contracted position 16, it returns to its rest position 18, thus completing the cycle. As a result of this vibrational contraction/expansion movement of the piezoelectric element, acoustic waves are generated through a series of vibrational cycles. In FIG. 1(b), the shear mode. piezoelectric element begins at rest 20. It then receives an electric signal, which causes it to shear to position 22. After the piezoelectric element has reached its fully sheared position 22, it will shear in the opposite direction towards its rest position 24. It continues to shear past the rest position 24 until it reaches a fully sheared position 26. Finally, after reaching its fully sheared position 26, it returns to its rest position 28, thus completing the cycle. As a result of this vibrational shearing movement of the piezoelectric element, acoustic waves are generated through a series of vibrational cycles.
In essence, acoustic touchscreens are simply bandpass filter systems. In other words, if a plurality of different frequency signals are input into the touchscreen, the touchscreen will only output a particular one of those signals. This particular signal will have a specific frequency which is known as the operating frequency of the touchscreen. For example, if a series of signals at between 1 and 10 MHz (1, 2, 3, etc.) are input into a particular touchscreen, the touchscreen will only output a signal at one of those frequencies, for example 5 MHz. This frequency (the operating frequency) is defined by the material of the touchscreen substrate (which defines the velocity of the signal through the substrate) and the spacing between reflective elements of a reflective array of the touchscreen (the spacing must be an integer multiple of the wavelength of the signal). Based upon the foregoing, the other elements of the touchscreen are designed for use at the operating frequency. Conventionally, associated electronics drive the touchscreen with tone bursts at this operating frequency, acoustic waves are generated, propagated and received at this operating frequency, and associated electronics process received electronic signals at this operating frequency.
Commercial acoustic touchscreen systems are typically designed having an operating frequency close to 5 MHz. Attenuation rates of acoustic waves increase rapidly with increasing frequency. For example, an operating frequency of 10 MHz would greatly reduce the maximum propagation distance, and hence limit the touchscreen to sizes too small for many applications of commercial interest, although higher operating frequencies may be useful for smaller touchscreens, such as those found in PDA, mobile phones, etc. On the other hand, use of a much lower operating frequency leads to larger acoustic wavelengths, stronger diffraction effects, and less well-directed acoustic beams. This means that wider reflective array borders are required. Ultimately, using lower operating frequencies leads to reduced touch position resolution. Thus, operating frequencies close to 5 MHz are the commercial standard.
In general, piezoelectric elements are designed to resonate at the operating frequency of the touchscreen system, to ensure acceptable levels of efficiency. In conventional acoustic touchscreen piezoelectric elements, the fundamental or first order thickness mode resonance of the piezoelectric element is at least approximately matched to the touchscreen system's operating frequency. This condition is equivalent to requiring that the piezoelectric element thickness be one-half of the bulk wave wavelength in the piezoelectric material. This is illustrated in FIG. 2, which shows the piezoelectric element thickness, T, for a first-order thickness resonance mode. This resonance condition determines the thickness of the piezoelectric elements according to the formula       T    =                  λ        2            =                        V                      2            ⁢            f                          =                  N          f                      ,where T is the piezoelectric element thickness, λ is the wavelength of the relevant bulk wave in the piezoelectric material, V is the speed of sound of the relevant bulk wave in the piezoelectric material, and f is the resonance frequency of the piezoelectric element. For convenience, manufacturers of piezoelectric elements often define half the speed of sound within the relevant piezoelectric material to be the frequency constant N.
Transducers designed to generate Rayleigh waves in a touchscreen substrate generally require a pressure mode piezoelectric element. For pressure mode piezoelectric elements, the relevant bulk wave is the bulk pressure wave. For a typical PZT material, the frequency constant for pressure-mode vibrations is typically in the neighborhood of N=2000 m*Hz. Thus, for a typical touchscreen operating frequency of approximately 5 MHz, this leads to a piezoelectric element thickness of about T=400 μm for pressure mode piezoelectric elements. While such thin slabs of piezoelectric ceramic material are rather fragile and easily broken, they are routinely manufactured and used in touchscreen assembly without serious problems, provided they are given proper care and handling.
The situation is much different, however, for shear mode piezoelectric elements. Transducers designed to generate horizontally polarized shear waves in a touchscreen substrate generally require a shear mode piezoelectric element. For shear mode piezoelectric elements, the relevant speed of sound is the bulk shear wave velocity in the piezoelectric material. Due to the dramatically slower velocity of shear waves relative to pressure waves, typical PZT frequency constants for shear mode vibrations are in the neighborhood of N=900 m*Hz. This leads to a piezoelectric element thickness of approximately T=180 μm, less than half the thickness of the corresponding pressure mode piezoelectric element.
The break strength of a slab varies with the square of its thickness. Therefore, since                     180        2                    400        2              ≈    0.20    ,a 180 μm thick shear mode piezoelectric element will break with about one-fifth of the force required to break a 400 μm thick pressure mode piezo. As a result, while 5 MHz pressure-mode PZT piezoelectric elements are strong enough for routine piezoelectric element manufacture and transducer assembly, the much weaker 5 MHz shear mode PZT piezoelectric elements are too fragile for these purposes. Therefore, while lithium niobate shear mode piezoelectric elements are more expensive then PZT elements, they provide better strength characteristics then PZT elements.
For a given frequency, lithium niobate piezoelectric elements are somewhat thicker and stronger than the corresponding PZT piezoelectric elements. Nevertheless, shear mode lithium niobate piezoelectric elements at 5 MHz are still very fragile. More importantly, mono-crystalline lithium niobate is a more expensive piezoelectric material than ferroelectric ceramic materials, such as PZT and lead titanate. In addition, PZT has stronger piezoelectric coupling constants than lithium niobate does. Therefore, were it not for the fragileness problem, PZT shear mode piezoelectric elements would be more beneficial than lithium-niobate piezoelectric elements in touchscreen applications.
As new applications develop for handheld computers with touchscreens, there may be market opportunities for smaller acoustic touchscreens with smaller sizes, higher operating frequencies, and hence thinner piezoelectric elements. This may lead to fragileness problems even for pressure-mode PZT piezoelectric elements.
Acoustic touch sensor applications need not be limited to transparent touchscreens placed in front of displays. Opaque sensors of various sizes and shapes may be considered. For example, in robotic applications, collision detection may be provided by tiling the exposed surfaces of a robot with touch sensors. The fragileness of thin PZT piezoelectric elements places undesired constraints on the choice of operating frequencies for such acoustic touch sensor systems.
Accordingly, there is a significant need to improve the design of acoustic transducers, particularly shear mode transducers, so that they can be made thicker, and thus more durable.